The main job of a CPU is to follow program instructions, but it can only process a little at a time. For the rest of the programs and data that it isn't using yet, it uses RAM to store it.

RAM (Random Access Memory)

RAM is the most common type of primary computer memory for temporary storage of program instructions and other data. RAM chips are divided into equally-sized memory locations, each with their own unique address so that the computer will be able to tell them apart when it needs to save or get information. Any piece of information can be stored in a RAM location so that you can randomly access what you need at any time, hence why it's called random access memory. However, since the information stored in RAM is merely a pattern of electrical current flowing through microscopic circuits in silicon chips, when a problem occurs or the electricity goes off, the computer forgets everything that was stored in RAM. Because it can only hold information temporarily, RAM is often called volatile memory.

ROM (Read-Only Memory)

Another type of memory exists that, unlike RAM, is nonvolatile and can store information without losing it, called ROM. In this type of memory, however, the computer can't write any new information; it can only read the information that is already on the ROM.